Without limiting the scope of the invention, its background is described in connection with the following: Antennas; and User Interference (Body Effect).
An antenna converts electrical current to and from electromagnetic waves. Antennas are passive devices that can couple mutually. An antenna's "gain" is based on its directivity and efficiency (lack of dissipative losses). Directivity is an antenna's ability to focus energy in a desired direction. If desired, energy can be focused into a narrow beamwidth. This focused energy gives the ability to communicate over great distances, but as the angle of the antenna changes, the direction of the beam also changes, thus requiring proper orientation of the antenna for a good signal.
An antenna is designed for maximal "gain" at a particular frequency. Conventionally this is accomplished by calculating the wavelength of the desired frequency and designing an antenna that is a fraction of the wavelength. Typical antennas are a quarter wavelength to 5/8ths of a wavelength long. Under the Global System for Mobile communication (GSM), which is standardized at approximately 900 MHz and 1800 MHz, a typical GSM mobile radio may need to operate in dual band mode. If the mobile radio has the capability for dual band operation, the antenna must be designed to operate at approximately 900 MHz (wavelength of 32.8 cm) and 1800 Mhz (wavelength of 15.6 cm). FIG. 1 shows a typical prior art example of a dual band antenna with quarter wavelength elements. Mobile radio 10 has an external dual band antenna 16 containing elements 12 and 14. Under the GSM system described above, antenna 14 would be about 3.9 cm for 1800 Mhz operation and antenna 12 about 8.2 cm for 900 Mhz operation. Of course, dual band antennas are applicable not just to GSM systems but also to Time Division Multiple Access (TDMA) systems and Code Division Multiple Access (CDMA) systems.
Antennas used with transmitting radios are often dipoles involving two elements: the radio case and an appendage to the radio case like a whip or helical wound whip. The combination of the case and the whip form an off-center-fed dipole. Above 800 MHz, the helix-radio combination is generally longer than a half-wave, so the radiation pattern no longer has a peak at the horizon. This results in a significant performance penalty and is the reason why a coaxial dipole is may be used at frequencies above 800 MHz.
Antennas that usually are associated with personal communication devices (PCD) are small, both electrically and physically. The communication devices are usually worn on the body or held in the hand. Due to the antenna's proximity to the user's body, the user often becomes part of the antenna system.
Internal antennas for personal communication devices offer an advantageous form factor that is convenient for slipping the device in a pocket or purse. External antennas increase the length of the PCD and often snag when the device is being removed from, or returned to, a pocket or purse. Unfortunately, when using an internal antenna in some situations, reception and transmission may be hindered by various factors, such as placement of the user's head or hand between the PCD and its base station.
The user's body causes a type of interference, known as the "body effect," that is of particular concern for mobile radio. Basically, the human body can be thought of as a large column of salt water that conducts electromagnetic waves away from the mobile radio's transceiver. In the case of cellular telephones, this effect is intensified because the user often covers part of the antenna with his hand when using the telephone. There is significant loss when the radio is held in the hand because the hand is wrapped around one of the antenna dipole elements. Many users rest their fingers on the external antenna when using the telephone, further increasing signal loss.
The standing human body behaves essentially like an inefficient wire antenna at frequencies below about 150 MHz. The body exhibits a whole-body resonance to vertical polarization that can contribute significantly to VHF radio system link margin. The erect body is resonant to vertically polarized incident fields in the range of about 40 to 80 MHz, depending on the presence and type of ground. That is, the body on a perfectly conducting ground looks like a quarter-wave element with a ground image, so its resonant length is about 3.4 m, while in free space the resonant length is 1.7 m. At frequencies of interest to cellular phone operation, the user can cause a drop in power of 5 to 20 dB, sometimes greater, by covering the antenna with his hand.
With the advent of electronic address books and schedulers, users of portable electronics, such as cellular telephones or laptop computers, often wish to exchange data with other electronic devices. For many years the standard solution was to connect a data cable between the devices. This method was inconvenient because it required the user of the portable device to carry a data cable.
A relatively recent innovation is a wireless data link. Essentially, the necessity of a data cable has been eliminated by use of wireless technology to transmit data. For consumer electronics, one of the most common wireless links is an infra-red optical link. They have become so popular that infra-red wireless links are included in some cellular telephones.
A great many mobile telephones have an infra-red data port even though they do not have infra-red communication capability. Many distinct models of mobile telephone use the same external casing in order to obtain efficiency of scale during manufacture. Because mobile telephone engines have different capabilities, not all engines need every feature of the external casing. For example, an engine that does not support an infra-red (IR) data link to a personal computer does not utilize the IR data port in its casing. A manufacturer may still sell this engine in the casing that has a data port because it is more economical than making a custom casing without the data port.
In summary, internal antennas for personal communication devices have the disadvantage that the user's body can interfere with reception and transmission of signals. External antennas have the disadvantage that the user often interferes with the signals by touching the antenna. As discussed above, touching or close proximity to the antenna can cause significant power loss. External antennas have the additional disadvantage that they snag easily and generally make a cellular telephone more cumbersome. Finally, many cellular telephones have infra-red data ports that they are not capable of utilizing.
Additional general background, which helps to show the knowledge of those skilled in the art regarding the system context, and of variations and options for implementations, may be found in the following: Harte, et al., GSM Superphones (1999); Lee, Mobile Cellular Telecommunications (1995); Siwiak, Radiowave Propagation and Antennas for Personal Communications (1998); Webb, Understanding Cellular Radio (1998); all of which are hereby incorporated by reference.